Pret A Manger and M&S’ commitment to using sustainably caught tuna in sandwiches is likely to lead to supply problems, as fishermen struggle to meet the sudden switch in demand.Earlier this month, both retailers announced that net-caught tuna would be replaced in their sandwiches by more sustainable pole-and-line caught fish, with Sainsbury’s, the Co-op and Morrisons also working to reduce the amount of net-caught tuna they sell.But according to tuna trading company Atuna, the sudden shift is “unrealistic” given the relatively small number of fishermen using pole-and-line methods.”Less than 10% of the world tuna catch is done by pole-and-liners nowadays. A large-scale shift to the method would implicate a major turnaround within the global tuna industry and would require major financial investment in training and personnel by fishing companies and the construction of new vessels fitted for pole-and-line fishing,” said Natalia Freitas, a spokesperson for Atuna. “Pole-and-line tuna already costs 50% more than net-caught, but if everyone decides to switch at once, the costs are likely to go even higher. Retailers will negotiate really hard with their suppliers because nobody wants to accept reduced margins and it will not affect small fishermen in a good way.”The switch by retailers to pole-and-line caught tuna was prompted by a new film – The End of the Line – which criticises net fishing for being unsustain-able. Environmental campaigners say that while stocks of skip- jack and yellowfin tuna are sus- tainable, indiscriminate ’purse seine’ net fishing results in a high level of by-catch, including endangered bluefin tuna, sharks and turtles.